I've attempted to do this (all weekend) on my P55A-UD3 and have not succeeded.
The closest I have gotten is seeing SLI enabled, however all processing hits GPU#2 and GPU#1 just hangs out. In most cases I don't even see SLI enabled.
The documentation indicates 2 methods and I have tried both of them. It clearly states "one or the other" however the ONLY time I get SLI to appear enabled is when both are config'd and that is when I get everything running on GPU#2.
Per the guide:
Q: I installed two videocards into motherboard, tried everything, but SLI don't appear
A: If you have old motherboard, based on mainstream chipsets like P965, P35, P45, P55 with lack of PCI-express lanes, you should check, which host feed PCI-Express slots for GPU's. For example if you have first PCI-E connected to northbridge PCI-E link, and second one connected to PCI-E from southbridge, then SLI can't be used. Graphics cards should have same host for SLI. For example, we tried three 8800 Ultra's on DFI UT P35 mainboard, but SLI enabled only for first two cards. On this motherboard, third GPU was connected to x1 PCI-Express of southbridge, no matter which slot size is. You can see this by looking pairs of traces from PCI-Express slot running to bridge.
Also check if SLIC certificate installed properly. To do this download latest version of Lavalys Everest diagnostic tool, and open section ACPI/DSDT there. You should have correct string in "SLI Certification". If there is "Not present" then try SLI patch install again.
They then provide a link to a screenshot of a system showing the DSDT edit
This is where my setup differs-- I NEVER see that DSDT edit no matter how many attempts or different methods that are used.
I was concerned that the configuration of my mobo may be causing the problem (1x pcie@x16 1x pcie@x4) but have found via the loveliness of google other users which have gotten this working on boards with identical lane layout, though admittedly I can't find a single person who has gotten this working on a P55A-UD3. I have attempted going back in BIOS versions (from F6 to F4) but this has not helped.
I have tried this on 2 Windows installs and have a backup for safekeeping so any suggestions no matter how silly/dangerous will be tried!